Description&#9;(Adapted from Applicant's Abstract): The bacterial cell surface is generally considered to be highly divergent from species to species. An exception to this rule is the expression of phosphorylcholine (ChoP). This unusual prokaryotic structure is now known to be exposed on the surface of the most common pathogens infecting the human respiratory tract; Haemophilus influenzae, mycoplasma, and Streptococcus pneumoniae. In addition, based on cross-reactivity to a MAb recognizing this structure, ChoP may be present on diverse phase-variable structures on N. meningitidis, N. gonorrhoeae, P. aeruginosa, and A. actinomycetemcomitans. We have defined the genetic basis of ChoP expression and the molecular mechanism controlling its phase variation in H. influenzae. This has allowed direct genetic analysis of clinical samples to show that the ChoP+ phase variants predominate on the mucosal surface of humans. The structure, however, is the target of innate immunity mediated by binding of C-reactive protein (CRP), which is bactericidal in the presence of complement. The focus of this proposal is to define the biological role of variants both with and without ChoP using H. influenzae as a prototype human respiratory tract pathogen. In Aim 1, we will determine whether switching to the ChoP- phenotype is required in natural H. influenzae infection (otitis media, pneumonia, bacteremia, and meningitis) to evade clearance by CRP and bactericidal anti-ChoP IgG. The ChoP phenotype in vivo will be determined by direct genetic analysis and compared to the local concentration of CRP and anti-ChoP antibody during infection. The local expression and concentration of CRP in the upper respiratory tract will be investigated. In Aim 2, we will determine how ChoP contributes to persistence on the mucosal surface. Genetically defined H. influenzae mutants with constitutive ChoP-on and ChoP-off phenotypes will be used to determine whether this host membrane-like structure contributes to (a) resistance to respiratory tract antibacterial peptides including LL-37 and tracheal antimicrobial peptide (TAP), and (b) colonization by functioning as a bacterial adhesin to host epithelial cells via putative ChoP ligands including GalNAcb 1-4Gal on the asialo-GM1 glycolipid and the platelet activating factor receptor. The blocking of complement mediated killing by naturally acquired secretory IgA recognizing ChoP will be explored as an explanation for the selection of the ChoP+ phenotype on the mucosal surface, despite the increased susceptibility of this phenotype to CRP and complement.